Voltage-dependent resistor

ABSTRACT

Voltage-dependent resistor comprising a one-grain-thick layer provided with electrode layers which are in contact with the grains and which are arranged in a pattern of islands partly overlapping each other and located on either side of the layer, so that mutually insulated parts of the grain layer can be connected in series.

[ 1 Apr. 10, 1973 United States Patent 1 Hagen [56] References CitedUNITED STATES PATENTS 10/1965 Johnson et [54] VOLTAGE-DEPENDENT RESISTOR[75] Inventor: Siegfried Hendrik Hagen, Emmasingel, Eindhoven,Netherlands ......29/621 [73] Assignee: U.S. Philips Corporation, New....338/20 6/1969 Armbruster.....................i..

3,210,831 Johnson York, N.Y.

[22] Filed:

Feb. 22, 1972 Primary Examiner-C. L. Albritton AttorneyFrank R. Trifari[21] Appl. No.: 228,095

Related US. Application Data [63] Continuation of Ser. No. 6,203, Jan.27, 1970,'aban- [57] ABSTRACT Voltage-dependent resistor com doned.prising a one-grainthick layer provided with electrode layers which arein [30] F i A li fl P i it D t contact with the grains and which arearranged in a pattern of islands partly overlapping each other and yer,so that mutually incan be connected in se- Feb. 1, 1969 Netherlands..690,1659

located on either side of the la sulated parts of the grain layer.338/20, 338/223, 338/325,

r 000 n %7 k 0 L U C G a m U I l. l. 2 l 5 5 1 .l

[58] Field of Search............ ...338/13, 20, 21, 100, 9 Claims, 5Drawing figures C le PATENTEBAPR 1 01973 SHEET 1 [IF 2 INVENTOR.

S H HAGEN AGEN PATENTED M01973 3,727, 165

sum 2 OF 2 Jill,"

INVENTOR. S. H. HAG EN VOLTAGE-DEPENDENT RESISTOR This is a continuationof application Ser. No. 6,203, filed Jan. 27, 1970, and now abandoned.

The invention relates to a voltage-dependent resistor comprising aone-grain-thick layer comprising grains, preferably semiconductivegrains, embedded in an electrically insulating binder and projecting oneither side from the binder, an electrode layer being applied to eitherside of said grain layer, said electrode layers being completelyseparated from each other by the grain layer and establishing contactswith the projecting parts of the grains.

Voltage-dependent resistors of the kind set forth are known, forexample, from US. Pat. No. 3,210,831. This Specification discloses avoltage-dependent resistor comprising a one-grain-thick layer of siliconcarbide grains embedded in a synthetic resin and projecting on eitherside therefrom. This grain layer is covered on either side with acoherent electrode layer. In operation a voltage difference is' appliedbetween these two electrode layers.

Such a known voltage-dependent resistor exhibits within a given voltagerange a very strong, non-linear current increase with an increasingvoltage. With voltage-dependent resistors having a one-grain-thick layerof the kind set forth this voltage range is restricted for a given grainlayer to comparatively low voltage values, for example, of the order ofa few volts.

The invention has for its object inter alia to provide a construction ofa voltage-dependent resistor in which by using a one-grain-thick layerthe operational voltage at which the resistor exhibits very useful,non-linear properties is considerably higher than in the known devicesdescribed above.

The invention is based inter alia on the recognition of the fact that byusing electrode layers of particular structure on a voltage-dependentresistor having a onegrain-thick layer of grains embedded in insulatingmaterial the operational voltage of the resistor can be materiallyraised in dependence upon the fastening area of the connectingconductors as compared with the value obtained in the known devices,whilst in addition tappings can be provided on the voltage-dependentresistor.

Therefore, according to the invention'a voltage-dependent resistor ofthe kind set forth is characterized in that each of the electrode layerscomprises a number of island-shaped regions arranged in one or morerows, at least one island of each row partially overlapping twoconsecutive islands belonging to a second row and located on theopposite side of the grain layer, whilst at least two islands belongingto said rows are provided with a connecting conductor so that betweenthese connecting conductors all parts of the grain layer located-betweenthe overlapping parts of the islands located between the connectingconductors are connected in series with each other.

The present invention provides inter alia the important advantage thatthe satisfactory, reproducible, nonlinear properties obtained in aone-grain-thick layer at a comparatively low voltage can be utilized forobtaining a very satisfactory, non-linear current-voltage characteristiccurve at higher voltages. By connecting in series, in accordance withthe invention, a plurality of one-grain-thick regions electricallyinsulated from each other in the direction of the layer a considerablybetter characteristic curve can be obtained than by using a layer morethan one grain thick because in a voltage-dependent resistor inaccordance with the invention junctions between grains having poorlyreproducible and undesirable electric properties do not occur.

A further important advantage is that tappings can be provided on avoltage-dependent resistor in accordance with the invention, which is ingeneral not possible with the known voltage-dependent resistorsdescribed above. For this purpose one or more of the further islandslocated between said connecting conductors may be provided with afurther connecting conductor. The connecting conductors may have theform of metal tracks applied to the grain layer, which may be connected,if desired, to an electrode system provided on the grain layer. In thisway composite circuit arrangements can be provided on a single grainlayer.

It should be noted that said rows of islands need not be rectilinear;they may form sequences of consecutive islands along curved or linesconsisting of consecutive linear segments in different directions. If anelectrode layer is formed by more than one row of islands, the seriescombination of grain layer portions in one row may be combined with theparallel connection of rows or of parts of rows with each other.

The grains may be made of silicon carbide or other suitable materials,preferably semiconductor materials, which together with the electrodelayers applied thereto from voltage-dependent resistors havingsatisfactory characteristic curves. Very satisfactory, non-linearcharacteristic curves are obtained by using grains consisting, forexample, of zinc-doped gallium phosphide or n-type silicon. The grainsmay be homogenous, in which case the non-linear current-voltagecharacteristic curve is obtained by the metalsemiconductor junctionsbetween grains and electrode layers. In a further preferred embodimentsemiconductor grains having a pnpor npn-structure are advantageouslyused, the two regions of the same conductivity type of which establish acontact with an electrode layer on opposite sides of the grain layer.Between electrode layers located on either side of the grain layer abreakdown voltage appears in both senses, the value of which isdetermined by the pn-junctions of the grains.

In a further preferred embodiment an even number of grain layer portionslocated between overlapping islands of a voltage-dependent resistor inaccordance with the invention are connected in series between twoconnecting conductors. This embodiment is particularly advantageous whenthe current-voltage characteristic curve measured between the twoelectrode layers located opposite each other on either side of the grainlayer is asymmetrical, which will be explained more fully hereinafter.

The invention will now be described more fully with reference to theexamples and the drawing, in which FIG. 1 is a schematic plan view of adevice having a voltage-dependent resistor in accordance with theinvention,

FIG. 2 is a schematic cross sectional view of the device of FIG. 1 takenon the line 11-,

FIG. 3 is a schematic cross sectional view of a detail of the sectionalview of FIG. 2,

FIG. 4 illustrates current-voltage characteristics measured on thedevice shown in FIGS. 1 to 3 and FIG. is a schematic cross sectionalview of a detail of a voltage-dependent resistor in accordance with theinvention, comprising grains having a pup-structure.

The Figures are schematic views and are not to scale and for the sake ofclarity particularly the dimensions in the direction of thickness arestrongly exaggerated. Corresponding'parts are designated in theFigures'by the same reference numerals.

The device shown in FIGS. 1 to 3' comprises a onegrain-thick layer 1comprising grains 2 of a thickness of 40 to 60 u (see FIG. 3'),consisting of gallium phosphide doped with 5.10" percent by weight ofzinc-The grains 2 are embedded in an electrically insulating binder 3 ofpolyurethane, the grains projecting on either side of the grain layerfrom the binder (see FIG. 3). The grain layer may be made for instanceby one of the methods disclosed in French Pat. specification No. 1 ,519,072.

The grain layer 1 has applied to it on one side an aluminum electrodelayer 4 of a thickness of about 1 t. On the opposite side a similarelectrode layer Sis applied. The electrode layers 4 and 5 are completelyseparated from each other by the grain layer 1 and are in contact withthe portions of the grains 2 projecting from the binder 3. I j v Theelectrode layers 4 and 5 (see FIG. l),comprise each two rowsofisland-shaped regions 4A, B etc. and 5A, B, etc. respectively. Thedimensions of the square islands are about 4.5 X 4.5 mms. In FIG. 1 theoutlines dicated by solid lines and those of the metal layers 5 locatedbeneath the grain layer are indicated by broken lines.

FIG. 2 is a sectional view of two rows of islands 4A to F and 5A to Flocated on opposite sides of the grain layer. Each one of the islands 48to 4F 'overlaps'partially two consecutive islands 5A to F belonging tothe second row on the opposite side of the grain layer. Con versely eachone of the islands 5A to E overlaps partially two of the oppositeislands 4A to F.v The islands 4A and SF. are provided with connectingconductors-6 and 7 (see FIG; 1). These connecting conductors are formedin this embodiment by aluminum layers located on the grain layer andconnected to the islands. The connecting conductors may, as analternative, be formed by connecting wires connected to an island andnot located on the grain layer. It will be apparent from the Figuresthat (see FIG. 2) between the connecting conductors 6 and 7 all parts 8of the grain layer V1 (in total 11) located between the overlappingparts of the islands 4A to SF situated between the'connecting conductors6 and 7 are connected in series with each other.

On each side of the grain layer the two rows of islands have one island(4A and SF respectively)-in common. Between the connecting conductors 6and 7 two sequences of series-connected regions 8 are thus connected inparallel with each other. When a voltage difference is appliedbetweenthe connecting conductors 6 and 7, the voltage difference acrosseach of the regions 8 is therefore approximately; one eleventh of thetotal voltage difference betweenthe connecting conductors6 and 7. I r

of the metal layers 4 applied to the grain layer 1 are in- FIG. 4 showsby way of comparison the current-voltage characteristic curve (a),measured between the connecting conductors 6 and 7, and thecurrent-voltage characteristic curve (b), measured across a singleregion 8 (FIG. 2) between opposite islands. It will be apparent that theoperational voltage may be any multiple of that obtained by avoltage-dependent resistor comprising only one region 8, in accordancewith the number of islands located between the conductors 6 and 7.

The non-linear resistance characteristics shown in FIG. 4 are obtainedowing to the non-linear contact junctions between the electrode layersand the gallium phosphide grains. An important advantage of the voltagedependent resistor according to the invention is that even when aplurality of one-grain-thick regions 8 are connected .in series theover-all resistance is determined substantially only by themetal-semiconductor junctions, whilst junctions between the grainsthemselves are avoided, which might adversely affect thereproducibility.

In the embodiment shown in FIGS. 1 to 3 (see FIG. 1) one of the islands4 located between the conductors 6 and 7 is provided with a furtherconnecting conductor 9, formed by a metal track. This conductor 9constitutes a tapping of the voltage-dependent resistor between theconductors 6 and 7 and isjconnected to a metal layer 10 associated witha further electrode system provided on the grain layer.- This electrodesystem is formed in this example by a further voltagedependent resistorformed by the metal layer 10, a metal layer 11 on the other side and theportion of the grain layer 1 located between the layers 10 and 11. Inthis way a composite circuitry, part of which is shown in FIGS. 1 to 3,is obtained on the same grain layer 1.

Not only gallium'phosphide, but also (preferably ntype conductive)silicon may advantageously be used for the grains,- in which case alsometalsemiconductor junctions of very satisfactory current-voltagecharacteristic curves can be obtained. Y

The device described may be manufactured by subjecting a one-grain-thicklayer having grains projecting on either side from the binder to an ionor electron bombardment, after which the aluminum layers 4 and 5 arevapor-deposited through a mask to form the various islands and metaltracks. After the formation of the contact between the grains and thealuminum layer by means of a short current pulse across the grain layerbetween the aluminum layers 4 and 5, the desired current-voltagecharacteristic curve is obtained. This is described in U.S. Pat. No.3,670,214 issued June 13, 1972. Then input and output wires areconnected with the suitable places, after which the assembly may bearranged in an appropriate envelope.

FIG. 5 is a schematic cross sectional view of a detail of a furtherembodiment of a voltage-dependent resistor according tothe invention.The non-linear current-voltage characteristic curve is obtained bymeansof pn-junctions in the grains. In this example the grains consistof silicon and have a highly doped p-type core 22, partly surroundedbyan n-type layer 26 and a ptype layer 27. The aluminum electrode layer24 constitutes a practically ohmic contact with the core 22 and thealuminum layer 25 establishes a practically ohmic contact with theoutermost p-type layer 27. The

voltage-dependent resistor is otherwise analogous to that shown in FIG.2. FIG. 5 only shows part of one of the grain layer regions 8 locatedbetween overlapping islands (see FIG. 2). i

In this embodiment the grains have each a pn-junction 28, locatedbetween the regions 22 and 26 and a pn-junction 29 between the regions26 and 27. Where these 'pn-junctions intersect the surface of thegrains, they are covered by parts 30 of the binder 23.

Since in general the pn-junctions 28 and 29 will not have the samebreakdown voltage, the use of a single coherent electrode layer oneither side of the grain layer would provide a voltage-dependentresistor having an assymmetric current-voltage characteristic curve. Animportant advantage of the invention in this embodiment is thereforethat when grains are used which comprise each an asymmetric npnorpnp-structure respectively a voltage-dependent resistor having asymmetric current-voltage characteristic curve can be obtained providedthis voltage-dependent resistor comprises a series combination of aneven number of regions 8 (see FIG. 2). This advantage also applies tohomogeneous grains with metal-grain junctions (Schottky junctions),having different current-voltage characteristic curves on either side ofthe layer.

The granular layer structure of FIG. 5 may be obtained on the basis ofp-type silicon grains, in which by methods generally used insemiconductor technology an n-type layer 26 and a p-type layer 27 arediffused.

Then in known manner a one-grain-thick layer is made from these grains,which project on either side from the binder 33, after which on one sideof the layer the cores 22 of the grains are exposed by etching forestablishing contacts with the electrode layers applied subsequently.

It will be obvious that the present invention is not restricted to theembodiments described above and that within the scope of the inventionmany variants are possible to those skilled in the art. For example,other grain materials and other materials for the electrode layers maybe used.

What is claimed is:

l. A voltage-dependent resistor comprising a onegrain-thick layer ofsemiconductor grains, embedded in an electrically insulating binder andprojecting on either side from the binder, an electrode layer beingapplied to either side of this grain layer in contact with projectinggrain parts, said electrode layers being completely separated from eachother by the grain layer and exhibiting between them a non-linearvoltage-current characteristic, each of the electrode layers comprisinga plurality of island-shaped regions arranged in at least one row, atleast one island of each row overlapping partially two consecutiveislands belonging to a second row on the opposite side of the grainlayer, while at least two islands belonging to said rows are providedwith a connecting conductor so that between these connecting conductorsall parts of the grain layer located between the overlapping parts ofthe islands located between the connecting conductors are connected inseries with each other.

2. A voltage-dependent resistor as claimed in claim 1, wherein thegrains have a pup-structure, the two regions of the same conductivitytype of said grains being in contact with an electrode layer on oppositesides of the grain la er.

A vol age-dependent resistor comprising a onegrain-thick layer ofsemiconductor grains, embedded in an electrically insulating binder andprojecting on either side from the binder, an electrode layer beingapplied to either side of this grain layer in contact with projectinggrain parts, said electrode layers being completely. separated from eachother by the grain layer and exhibiting between them a non-linearvoltage-current characteristic, each of the electrode layers comprisinga plurality of island-shaped regions arranged in at least one row, atleast one island of each row overlapping partially two consecutiveislands belonging to a second row on the opposite side of the grainlayer, while at .least two islands belonging to said rows are providedwith a connecting conductor so that between these connecting conductorsall parts of the grain layer located between the overlapping parts ofthe islands located between the connecting conductors are connected inseries with each other, while between two connecting conductors an evennumber of parts of the grain layer located between overlapping islandsare connected in series.

4. A voltage-dependent resistor as claimed in claim 1-, wherein thegrains have a npn structure, the two regions of the same conductivitytype of said grains being in contact with an electrode layer on oppositesides of the grain layer.

5. A voltage-dependent resistor of claim 1 wherein at least one of theislands located between said connecting conductors is provided with afurther connecting conductor.

6. A voltage dependent resistor of claim 5 wherein at least one of theconnecting conductors is formed by a metal track applied to the grainlayer.

7. A voltage-dependent resistor of claim 6 wherein said metal track isconnected to an electrode system provided on the grain layer.

8. A voltage-dependent resistor of claim 1 wherein the grains consist ofgallium phosphide.

9. A voltage-dependent resistor of claim 1 wherein the grains are madeof silicon, preferably n-type conductive silicon.

1. A voltage-dependent resistor comprising a one-grain-thick layer ofsemiconductor grains, embedded in an electrically insulating binder andprojecting on either side from the binder, an electrode layer beingapplied to either side of this grain layer in contact with projectinggrain parts, said electrode layers being completely separated from eachother by the grain layer and exhibiting between them a non-linearvoltage-current characteristic, each of the electrode layers comprisinga plurality of island-shaped regions arranged in at least one row, atleast one island of each row overlapping partially two consecutiveislands belonging to a second row on the opposite side of the grainlayer, while at least two islands belonging to said rows are providedwith a connecting conductor so that between these connecting conductorsall parts of the grain layer located between the overlapping parts ofthe islands located between the connecting conductors are connected inseries with each other.
 2. A voltage-dependent resistor as claimed inclaim 1, wherein the grains have a pnp-structure, the two regions of thesame conductivity type of said grains being in contact with an electrodelayer on opposite sides of the grain layer.
 3. A voltage-dependentresistor comprising a one-grain-thick layer of semiconductor grains,embedded in an electrically insulating binder and projecting on eitherside from the binder, an electrode layer being applied to either side ofthis grain layer in contact with projecting grain parts, said electrodelayers being completely separated from each other by the grain layer andexhibiting between them a non-linear voltage-current characteristic,each of the electrode layers comprising a plurality of island-shapedregions arranged in at least one row, at least one island of each rowoverlapping partially two consecutive islands belonging to a second rowon the opposite side of the grain layer, while at least two islandsbelonging to said rows are provided with a connecting conductor so thatbetween these connecting conductors all parts of the grain layer locatedbetween the overlapping parts of the islands located between theconnecting conductors are connected in series with each other, whilebetween two connecting conductors an even number of parts of the grainlayer located between overlapping islands are connected in series.
 4. Avoltage-dependent resistor as claimed in claim 1, wherein the grainshave a npn structure, the two regions of the same conductivity type ofsaid grains being in contact with an electrode layer on opposite sidesof the grain layer.
 5. A voltage-dependent resistor of claim 1 whereinat least one of the islands located between said connecting conductorsis provided with a further connecting conductor.
 6. A voltage dependentresistor of claim 5 wherein at least one of the connecting conductors isformed by a metal track applied to the grain layer.
 7. Avoltage-dependent resistor of claim 6 wherein said metal track isconnected to an electrode system provided on the grain layer.
 8. Avoltage-dependent resistor of claim 1 wherein the grains consist ofgallium phosphide.
 9. A voltage-dependent resistor of claim 1 whereinthe grains are made of silicon, preferably n-type conductive silicon.